Extremely high liquid barrier fabrics

ABSTRACT

One embodiment of the present invention is a nonwoven fabric comprising a support web and a fibrous barrier web, having a hydrohead of at least about 145 cm and a Frazier permeability of at least about 0.3 m 3 /m 2 -min.

[0001] This invention relates to nonwoven fibrous structures and moreparticularly to breathable fabrics and sheet structures formed by fiberswhich are held together without weaving or knitting.

[0002] Nonwoven fibrous structures have existed for many years and todaythere are a number of different nonwoven technologies in commercial use.To illustrate the breadth of nonwoven technologies, paper is probablyone of the earliest developed nonwoven fibrous structures. Nonwoventechnologies continue to be developed by those seeking new applicationsand competitive advantages. One broad market area that has proven to behighly desirable because of its large volume and economics is theprotective apparel market. This market comprises protection fromhazardous chemicals such as in chemical spill clean up, from liquidssuch as blood in the medical field and from dry particulates or otherhazards such as painting or asbestos removal.

[0003] It is known that for a garment to be comfortable, it mustaccommodate the body's physiological need for thermal regulation. Inwarm environments, heat energy must be expelled from the body. This isdone principally by a combination of direct thermal conduction of heataway from the body through the fabric and air layers at the skinsurface, convection of heat away from the body by flowing air, and bythe cooling effects of evaporation of sweat from the surface of theskin. Clothing which appreciably inhibits heat transfer can cause heatand moisture buildup and this can result in discomfort due to warm,sticky, clammy and or sweaty sensations. In the extreme case, forexample, where protective clothing prevents adequate thermal regulationduring activity in a warm and humid environment, such clothinglimitations not only lead to discomfort, but can result inlife-threatening heat stress. For this reason, frequently, clothinglimitations impose limitations on activity to avoid the consequences ofheat stress.

[0004] Studies have shown that the most comfortable garments with theleast restrictions on physical activity in warm, humid environments, arethose most able to breathe through mechanisms of air exchange with theenvironment. (Bernard, T. E., N. W. Gonzales, N. L. Carroll, M. A.Bryner and J. P. Zeigler. “Sustained work rate for five clothingensembles and the relationship to air permeability and moisture vaportransmission rate.” American Industrial Hygiene Conference, Toronto,June 1999; N. W. Gonzales, “Maximum Sustainable Work for Five ProtectiveClothing Ensembles and the Effects of Moisture Vapor Transmission Ratesand Air Permeability” Master's Thesis, College of Public Health,University of South Florida, December 1998).

[0005] Physical activity flexes fabric and garment. If a fabric has lowenough resistance to air flow, this, in turn, produces a pumping actionwhich pushes and pulls air back and forth through the fabric. By thismechanism, the exchange of warm moisture laden air within the garmentwith ambient air provides a significant cooling effect. Tests ofprotective garments made of the same cut, but with widely differing airflow resistance under warm humid conditions (32° C., 60% RH), have shownthat the garments made of fabrics with the least air flow resistancerepeatedly allowed subjects to achieve higher levels of activity withoutincurring heat stress. Conversely, garments made of fabrics with thehighest air flow resistance limited the physical activity of the samesubjects to the lowest levels to avoid heat stress. Garments made offabrics having intermediate air flow resistance allowed subjects toachieve intermediate levels of activity without heat stress. Theintermediate activity levels correlated very well with the air flowresistance of the fabric.

[0006] Clearly, under conditions where the body must transfer heat andmoisture to maintain comfort or avoid heat stress, it is desirable tofor garments to be made with fabrics having low air flow resistance.

[0007] Clothing provides protection from hazards in the environment. Thedegree of protection clothing imparts is dependent upon theeffectiveness of the barrier characteristics of the clothing. Where thefunction of the barrier is to keep environmental particulates or fluidsfrom penetrating a garment to reach the wearer, barrier is easilycorrelated with fabric pore size. The most effective barriers generallyhave the smallest pore size.

[0008] Unfortunately, smaller pore size also generally results in higherair flow resistance. In the studies cited above, the garments with thehighest barrier properties had the lowest airflow permeability and viseversa. So the ability to provide effective barrier protection inclothing and the ability to provide low air flow resistance, i.e., highair flow permeability, in the same garment are inversely related.

[0009] Hydrostatic head or “hydrohead” (AATCC TM 127-194) is aconvenient measure of the ability of a fabric to prevent waterpenetration. It is presented as the pressure, in centimeters of watercolumn (cmwc) required to force liquid water through a hydrophobicfabric. It is known that hydrohead depends inversely on pore size. Lowerpore size produces higher hydrohead and higher pore size produces lowerhydrohead.

[0010] Fabric air flow permeability is commonly measured using theFrazier measurement (ASTM D737). In this measurement, a pressuredifference of 124.5 N/m² (0.5 inches of water column) is applied to asuitably clamped fabric sample and the resultant air flow rate ismeasured as Frazier permeability or more simply as “Frazier”. Herein,Frazier permeability is reported in units of m³/m²-min. High Frazier,corresponds to high air flow permeability and low air flow resistancewhile low Frazier corresponds to low air flow permeability and high airflow resistance.

[0011] Microporous films have been used in barrier materials to achieveextremely high hydrostatic head liquid barrier properties, but at theexpense of breathability, such that their Frazier permeabilities areunacceptably low, rendering fabrics containing such films uncomfortablefor the wearer.

[0012] Currently, most melt-spun fibers have diameters on the order ofseveral tens of micrometers, whereas melt-blown fibers are known to havefiber diameters on the order of from about 1 to 10 micrometers.Recently, many researchers have made efforts to decrease fiber sizes inorder to obtain different benefits, as compared to conventional fibers.

[0013] Advances have been made in providing both high hydroheadproperties and high Frazier properties in the same fabric. For example,U.S. Pat. No. 5,885,909 discloses low or sub-denier nonwoven fibrousstructures which demonstrate an unusual combination of high Frazierpermeability and high hydrostatic head liquid barrier properties.

[0014] More recently, efforts have centered around obtaining fiberdiameters in the ‘nanofiber’ range, i.e. with diameters on the order ofless than about 0.5 micrometers (500 nm). However, production of suchsmall fibers has presented many problems including low throughput, poorefficiency in spinning and difficulties in fiber collection.

[0015] Conventionally, nanofibers have been produced by the technique ofelectrospinning, as described in “Electrostatic Spinning of AcrylicMicrofibers”, P. K. Baumgarten, Journal of Colloid and InterfaceScience, Vol. 36, No. 1, May, 1971. According to the electrospinningprocess, an electric potential is applied to a drop of a polymer insolution hanging from a metal tube, for example a syringe needle, whichresults in elongation of the drop of the solution to form very finefibers which are directed to a grounded collector. Fibers with diametersin the range of 0.05 to 1.1 micrometers (50 to 1100 nm) are reported. Anexample of a suitable electrospinning apparatus for forming thenanofiber-containing fabrics of the present invention is disclosed inU.S. Pat. No. 4,127,706, incorporated herein by reference.

[0016] The vast majority of investigations into nanofiber productionreported in the prior art literature have been directed to formation ofessentially hydrophilic polymer nanofibers, such as polyamide,polyurethane and the like. While some investigators have suggested thatnanofibers could be produced from hydrophobic polymers, few actualexamples of such hydrophobic nanofibers are disclosed in the literature.U.S. Pat. No. 4,127,706 discloses production of porous fluoropolymerfibrous sheet, suggesting the production of PTFE fibers with diametersin the range of 0.1 to 10 micrometers, but exemplifying only fibers withdiameters of 0.5 micrometer and above.

SUMMARY OF THE INVENTION

[0017] One embodiment of the present invention is a nonwoven fabriccomprising a support web and a fibrous barrier web, having a hydroheadof at least about 145 cm and a Frazier permeability of at least about0.3 m³/m²-min.

[0018] Another embodiment of the present invention is a hydrophobicnonwoven fabric comprising at least one support web and a barrier webwith fibers having diameters of less than 2.0 micrometers, a hydroheadof at least about 145 cm and a Frazier permeability of at least about0.3 m³/m²-min.

[0019] Another embodiment of the present invention is a nonwoven fabriccomprising a fibrous barrier web, said fabric having a hydrohead of atleast about 145 cm and a Frazier permeability of at least about 0.3m³/m²-min and having a relationship between barrier web basis weight,and fabric hydrohead and Frazier permeability described by the formula:${{Bwt}\left( {g\text{/}m^{2}} \right)} \leq \frac{4000 \cdot c \cdot \left( {1 - {2.3 \cdot c}} \right) \cdot \rho_{f}}{{Frazier} \cdot {Hydrohead}^{k{(c)}}}$

[0020] wherein ρ_(f), is the density of the barrier fibers, kg/m3, c isthe solids volume fraction of the barrier web, k(c)=3.58·c²−1.32·c+1.77,Frazier is in units of m³/m²-min, and hydrohead is in units ofcentimeters of water column.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a log/log plot of barrier properties of various priorart fabrics.

[0022]FIG. 2 is a reproduction of FIG. 1 with a plot of the line ofEquation 10 laid thereon.

[0023]FIG. 3 is reproduction of FIG. 1 with a plot of data from Equation14 wherein basis weight is maintained as a constant, and fiber size isreduced.

[0024]FIG. 4 is a plot of basis weight v. liquid barrier at constant airpermeability (Frazier).

[0025]FIG. 5 is a reproduction of FIG. 3 with a plot of data fromEquation 14 wherein air permeability (Frazier) is maintained as aconstant, and fiber size is reduced.

[0026]FIG. 6 is an illustration of the structure of the nonwoven fabricsof the present invention presenting a barrier to the advance of a liquidsurface.

[0027]FIG. 7 is a graphical presentation illustrating the relationshipof Equation 16 wherein achievable hydrohead as a fraction of potentialhydrohead is dependent upon D_(fS)/D_(fL) and GPD×Bwt.

DETAILED DESCRIPTION OF THE INVENTION

[0028] Unless otherwise specified, references to fiber diameters hereinare intended to be directed to the number average fiber diameter of thefibers.

[0029]FIG. 1 illustrates the inverse relationship between airpermeability and hydrohead for three sets of data. The first set istaken from U.S. Pat. No. 5,585,909, the second presents data measured onsamples of melt-blown nonwoven fabric, the third presents data measuredon three commercial nonwoven products: K-C Ultra® unreinforced surgicalgown, available from Kimberly Clark Health Care, Roswell, Ga.; Trimax®unreinforced surgical gown and DuPont Sontara® Optima® unreinforcedsurgical gown, both available from Allegiance Health Care, Mc Gaw Park,Ill.

[0030] It is of note in FIG. 1, that commercial nonwoven products haveair permeabilities in the range of woven fabrics. By way of reference, atightly woven polyester fabric (basis weight 95 g/m²) used in thetesting described above had a Frazier value of about 0.5 m³/m²-min,while, ASTM D737-96 lists the Frazier values for a sampling of severalwoven fabrics in the range of 2.5 to 66 m³/m²-min.

[0031]FIG. 1 shows that nonwoven barrier fabrics have a hydroheadtypically lower than about 100 centimeters of water column. The forcingpressure difference, ΔP, across the fabric can be related to theequivalent capillary radius, R of the largest pore water will penetrate,using the Washburn equation: $\begin{matrix}{{\Delta \quad P} = {{- \frac{2\sigma}{R}}{Cos}\quad {\theta.}}} & \left( {{Equation}\quad 1} \right)\end{matrix}$

[0032] Here σ is the surface tension of water (0.072 N/m) and θ is thewetting angle, i.e., the angle of intersection of the fluid surface withthe solid surface. For ΔP in units of centimeters of water column and Rin micrometers and assuming an ideally nonwetted surface (θ=180°),$\begin{matrix}{{\Delta \quad {P({cmwc})}} = {\frac{1468}{R({microns})}.}} & \left( {{Equation}\quad 2} \right)\end{matrix}$

[0033] From which it is concluded that hydrohead lower than about 100cmwc in FIG. 1 corresponds to largest pores of radius, R≧15 micrometers.

[0034] The Washburn relationship shows that to create better liquidbarriers which can withstand higher fluid pressures, fabric pore sizemust be reduced. Better liquid barrier fabric would be of benefit inmany applications including protective apparel. For example, in responseto concern about contamination with blood-borne pathogens, ASTM F1670specifies that an acceptable fabric must prevent penetration ofsynthetic blood (σ=0.042 N/m versus 0.072 N/m for water) at a pressureof 13800 N/m² (141 cmwc). From Equation 1, for a fabric to pass thistest (wetting angle θ=180°), the maximum fabric pore radius must be lessthan about 6 micrometers.

[0035] Microporous films with pores radii typically less than 1micrometer satisfy this criterion. Such films can be effective liquidbarriers, but they are very impermeable to air flow as well. Typicalmicroporous film air permeabilities, e.g., in the range of Frazier<0.008m³/m²-min, are too low to provide effective air exchange in a protectivegarment. This often leads to heat buildup and discomfort. In theextreme, it can even impair or limit work performance.

[0036] Fibrous porous media are inherently more permeable thanmicroporous films and a good choice for protective fabrics, but therelationship of FIG. 1 shows, in general, that significant increases inbarrier function resulting from reduced pore size will alsosignificantly reduce air permeability.

[0037] To understand the requirements for a nonwoven fibrous fabric tohave both high liquid barrier and high air permeability, it is useful toconstruct an analytical model of the fabric structure. Hydrohead as ameasure of liquid barrier is related to pore size as discussed above,and pore size is determined by structural characteristics of the fabric,including fiber size and void fraction. Fabric air permeability is alsodetermined by fundamental structural characteristics, including fibersize, void fraction and basis weight.

[0038] Pore Size:

[0039] The size of the pore space between fibers in a random fiber web,is proportional to fiber diameter, D_(f), as a determinant of the numberof fibers which can occupy a space. It is inversely proportional to thesolids volume fraction, c, which is the ratio of web volume occupied byfibers to the total web volume (i.e., (1—void fraction)). For metalfiber filters, Goeminne, et al, (“The Geometrical and FiltrationCharacteristics of Metal Fiber Filters—A Comparative Study”, Filtrationand Separation, Vol. 11, No. 4, pp 350-355 (1974)) report that themaximum pore diameter, D_(ρ) is described by: $\begin{matrix}{{Dp} = {\frac{D_{f}}{c}.}} & \left( {{Equation}\quad 3} \right)\end{matrix}$

[0040] An independent analysis of the stochastic structure of idealrandom fibrous webs for this work gives: $\begin{matrix}{{Dp} = {\frac{3 \cdot \pi \cdot D_{f}}{8 \cdot c}.}} & \left( {{Equation}\quad 4} \right)\end{matrix}$

[0041] Equation 4 predicts slightly larger maximum pore size thanEquation 3. Combining Equation 4 with Equation 2 provides a conservativeestimate of random web hydrohead in terms of fiber size and solidsfraction as: $\begin{matrix}{{\Delta \quad {P({cmwc})}} = {\frac{2493 \cdot c}{D_{f}({microns})}.}} & \left( {{Equation}\quad 5} \right)\end{matrix}$

[0042] Equation 5 is used for the results below.

[0043] Air Permeability:

[0044] Davies has presented a careful and well attested correlation offlow rate, pressure drop, fiber size and solids fraction on pads made ofa wide variety of fibrous materials. (Davies, C. N., “The Separation ofAirborne Dust and Particles,” The Institution of Mechanical EngineersProceedings (B), Nos. 1-12, Vol 1B, p 185, 1952-53) In terms ofdefinitions above, this correlation gives volumetric flow rate, Q, perunit flow area, A, as: $\begin{matrix}{{\frac{Q}{A} = \frac{\Delta \quad {P \cdot D_{f}^{2}}}{h \cdot \eta \cdot {f(c)}}},} & \left( {{Equation}\quad 6} \right)\end{matrix}$

[0045] where,

ƒ(c)=64·c ^(1.5)·(1+56·c ³)  (Equation 7)

[0046] Here ΔP is the pressure drop across the fibrous pad of thickness,h, and η is the viscosity of the flowing fluid. The Davies correlationis valid for 0.006<c<0.3 when the flow around fibers in the medium islaminar.

[0047] The thickness of the fibrous medium is related to the basisweight (Bwt) of the medium, the fiber density, ρ_(f), and the solidsfraction as follows: $\begin{matrix}{h = {\frac{Bwt}{\rho_{f} \cdot c}.}} & \left( {{Equation}\quad 8} \right)\end{matrix}$

[0048] Combining Equations 6 and 8, gives: $\begin{matrix}{\frac{Q}{A} = {\frac{{\rho_{f} \cdot \Delta}\quad {P \cdot D_{f}^{2} \cdot c}}{{Bwt} \cdot \eta \cdot {f(c)}}.}} & \left( {{Equation}\quad 9} \right)\end{matrix}$

[0049] Taking hydrohead to be the forcing pressure, ΔP, of Equation 5,the relationship between hydrohead and fiber size of Equation 5 can becombined with the above relationship between Q/A and fiber size to give$\begin{matrix}{\frac{Q}{A} = {\frac{6.2 \times {10^{6} \cdot \rho_{f} \cdot \Delta}\quad {P \cdot c^{3}}}{{Bwt} \cdot \eta \cdot {Hydrohead}^{2} \cdot {f(c)}}.}} & \left( {{Equation}\quad 10} \right)\end{matrix}$

[0050] If the flow forcing pressure difference, ΔP, of Equation 10 isset equal to 124.5 N/m², and consistent units are used, Q/A iscalculated directly as Frazier in units of cubic meter per square meterper minute (m³/m²-min). FIG. 2 shows that for typical polypropylenefabrics, Bwt=34 grams/m², c=0.1, and ρ_(f),=920 kg/m³, the model ofEquation 10 reasonably fits the data of FIG. 1 accounting for thegeneral trend.

[0051] Two further refining effects must be taken into account. First,thermal bonding (which is almost always necessary in the production ofnonwoven fabrics) at bond points which comprise a bonded area fraction,f_(ba), will reduce Q/A by the factor (1−f_(ba)), hence, $\begin{matrix}{\frac{Q}{A} = {\frac{{\rho_{f} \cdot \Delta}\quad {P \cdot D_{f}^{2} \cdot c}}{{Bwt} \cdot \eta \cdot {f(c)}} \cdot {\left( {1 - f_{ba}} \right).}}} & \left( {{Equation}\quad 11} \right)\end{matrix}$

[0052] Second, for the fabrics with fiber sizes less than about 5micrometers, air flow is known to “slip” past the fibers withoutencountering the full effects of viscous drag. The slip effect increasesas fiber size decreases. The effect is to increase flow at a givepressure drop over that predicted by Equation 10. Chmielewski and Goren(“Aerolsol Filtration With Slip Flow”, Environmental Science andTechnology, Vol. 6, No. 13, p 1101, 1972) have presented a correctionfactor for the case of slip flow through fibrous fabrics. The correctionfactor, here defined as S(c, N_(kn)) varies with solids volume fraction,c, and with the Knudsen Number, N_(kn), defined as $\begin{matrix}{{N_{kn} = \frac{2.48 \cdot \lambda}{D_{f}}},} & \left( {{Equation}\quad 12} \right)\end{matrix}$

[0053] where λ is the mean free path for collisions between airmolecules. Here, λ is taken to be 0.065 micrometers. For this work, thegraphical presentation of Chmielewski and Goren was fit very wellempirically with the function $\begin{matrix}{{S\left( {c,N_{kn}} \right)} = {\frac{1 + \frac{\left( {1.662 + {19.66 \cdot c} - {47.027 \cdot c^{2}}} \right)}{N_{kn}}}{1 + {{.9489} \cdot N_{kn}}}.}} & \left( {{Equation}\quad 13} \right)\end{matrix}$

[0054] The slip correction is incorporated in the flow model which thenbecomes: $\begin{matrix}{\frac{Q}{A} = {\frac{{\rho_{f} \cdot \Delta}\quad {P \cdot D_{f}^{2} \cdot c}}{{Bwt} \cdot \eta \cdot {f(c)}} \cdot \left( {1 - f_{ba}} \right) \cdot {{S\left( {c,N_{kn}} \right)}.}}} & \left( {{Equation}\quad 14} \right)\end{matrix}$

[0055] As above, if the forcing pressure drop across the fabric, ΔP, is124.5 N/m² (12.7 mm of water column), and η is the viscosity of air atroom temperature, and if consistent units are used, then Q/A is theFrazier permeability, here denoted in units of m³/m²-min.

[0056] The present inventor has determined that the model for hydrohead,Equation 5, and the model for flow, Equation 14, can be used together todefine the requirements for functionally superior liquid barrierfabrics. If the fabric is a multi-layer fabric, the model can be usedfor each layer to determine the properties of each, then the individuallayer properties can be combined to determine composite sheetproperties. For example, in a layered fabric, hydrohead is taken to bethe maximum hydrohead of any layer in the fabric. Air permeability isobtained from the relationship: $\begin{matrix}{\frac{1}{{Frazier}_{TotalFabric}} = {\sum\limits_{{Layer} = 1}^{j}{\frac{1}{{Frazier}_{{Layer} - j}}.}}} & \left( {{Equation}\quad 15} \right)\end{matrix}$

[0057] Models:

[0058] Model 1: Constant Basis Weight as Fiber Size is Decreased toIncrease Liquid Barrier.

[0059] For the case of a polypropylene fabric where Bwt=33.9 g/m²,f_(ba)=0, c=0.1, and ρ_(f)=920 kg/m³, the model provides the results ofTable 1, and FIG. 3. TABLE 1 Fiber Diameter Hydrohead FrazierPermeability (micrometers) (cmwc) (m³/m²-min) 2.0 125 2.58 1.5 166 1.541.0 249 0.75 0.7 356 0.41 0.5 499 0.23 0.3 831 0.10

[0060] The results of Model 1 illustrate the detrimental decrease offabric permeability when liquid barrier is increased by decreasing fibersize alone, without a decrease in basis weight.

[0061] Model 2: Constant Air Permeability (Frazier) as Fiber Size isDecreased to Increase Liquid Barrier.

[0062] For the case of a polypropylene fabric where Frazier=10m³/m²-min, f_(ba)=0, c=0.1, and ρ_(f)=920 kg/m³, the model provides theresults of Table 2, and FIGS. 4 and 5. TABLE 2 Hydrohead Basis Weight(cmwc) (g/m²) 125 8.761 166 5.213 249 2.553 356 1.386 499 0.788 8310.339

[0063] From FIGS. 4 and 5, it is clearly seen that the basis weight ofthe barrier layer must be decreased dramatically to maintain airpermeability as liquid barrier in terms of hydrohead is increased.

[0064] Problem of High Barrier and Thin Barrier Web:

[0065] In the extreme case of high permeability or high liquid barrieror both, the mechanical strength of the barrier layer can pose apractical limit the barrier level achieved. FIG. 6 shows a liquidinterface advancing against a fibrous barrier layer. The barrier layerconsists of a layer of small fibers with characteristically small poressupported by a layer of large fibers with characteristically largerpores. The pressure required to force a nonwetting fluid through thesmall pores of the barrier layer is given by Equation 5. This pressureforce is distributed across all the small fibers of the barrier layer.Hence, the loading of a representative small fiber, e.g., Fiber AB, isreadily obtained as a force per unit length. The span over which thesmall fiber must carry the pressure load is determined by the pore sizeof the support layer as given by Equation 4. If the span is too great,the tension in the small fibers can exceed the strength of the fibers,causing them to break.

[0066] In this case, the hydrohead is limited by a relationship betweenthe strength of the barrier fiber, and the basis weight of the barrierfiber layer, which determine the strength of the barrier layer, and thepore size of the support layer which determines the force load on thebarrier fibers. A relationship can be developed between the maximumforce load a barrier fiber can sustain just before breaking and theforce loading that the barrier fiber would have to sustain to achievemaximum hydrohead. If it is assumed that the barrier layer fiber loadingand geometry are microscopic equivalents to the uniform loading of amacroscopic cable strung between two supports and if it is assumed thata small barrier fiber deflects a distance equivalent to one largesupport layer fiber diameter before it breaks, the analysis of thisrelationship as a cable problem gives: $\begin{matrix}{{\frac{{Hydrohead}_{act}}{{Hydrohead}_{\max}} = {613 \cdot \frac{D_{fS}}{D_{fL}} \cdot \frac{{{GPD} \cdot {Bwt}}\quad \left( {g\text{/}m^{2}} \right)}{\sqrt{1 + {0.0867 \cdot \frac{\left( {D_{fL}/D_{fS}} \right)^{2}}{c^{2}}}}}}},} & \left( {{Equation}\quad 16} \right)\end{matrix}$

[0067] (Higdon, A., Stiles, W. B., Engineering Mechanics Statics andDynamics, Vector Edition, Prentice-Hall, 1962).

[0068] Here, Hydrohead_(act) is the hydrohead actually achieved.Hydrohead_(max) is the maximum hydrohead the barrier layer can achieve,given by Equation 5. D_(fS) and D_(fL) are the diameters of the smallbarrier layer fibers and the large support fibers respectively. GPD isthe tensile strength of the barrier layer fibers in grams per denier.Bwt is the basis weight of the barrier layer. The solids volume fractionis c.

[0069] Model 3: Illustration of the Problem of Low Barrier LayerStrength When Barrier Layer Basis Weight is Reduced to Maintain AirPermeability

[0070] If the barrier layer of Model 2 consisting of polypropylene(ρ_(f)=920 kg/m³) fibers of diameter D_(fS)=0.6 micrometers (Frazier=10m³/m²-min, c=0.1, GPD=1 gram per denier, and Bwt=1 g/m²) is laminated toa support layer with fibers of diameter D_(fL)=12 micrometers, thenEquation 16 gives: $\begin{matrix}{\frac{{Hydrohead}_{act}}{{Hydrohead}_{\max}} = {0.52.}} & \left( {{Equation}\quad 17} \right)\end{matrix}$

[0071] The maximum potential hydrohead for the barrier layer as obtainedfrom Equation 5 is 415 cmwc, but at a basis weight of 1 g/m², the layeris strong enough to withstand only about half of that pressure beforecollapsing. The maximum hydrohead could be realized by doubling thebasis weight of the barrier layer, but doubling the basis weight wouldreduce the air permeability of the composite fabric by half. There wouldbe an economic penalty as well for the higher basis weight.

[0072] An alternative solution is to reduce the pore size of the supportlayer by reducing the support layer fiber size. Per FIG. 7, theHydrohead_(act)/Hydrohead_(max) curve for GPD×Bwt=1, reaches unity whenthe ratio D_(fS)/D_(fL)=0.075. So the maximum hydrohead possible can berealized if the support layer fiber diameter is reduced to about 8micrometers. If the basis weight of such a support layer is less thanabout 9 grams/m², per Equation 7, the Frazier air permeability, is stillabout 10 m³/m²-min.

[0073] The model relationships presented here permit the rational designof fabrics for various balances of barrier and air permeability.Clearly, the underlying physics allow only certain balances ofproperties to exist. Once a realizable balance is specified, choices canbe made as to how to create a given balance.

[0074] For example, since permeability depends upon the square of fiberdiameter, choosing the largest fiber size consistent with achieving adesired barrier might be preferred as a means of achieving the highestpermeability. Hydrohead can be increased by calendering the fabric toincrease solids fraction (Equation 5). The dependence of hydrohead andFrazier on solids fractions is such that calendering the barrier layerto increase the solids fraction will increase barrier more than itdecreases Frazier. If smaller fiber size is selected for barrier as theproduct basis, basis weight can be adjusted within bounds to achieve thedesired air permeability. Other such tradeoffs can be assessed based oneconomics and the practicalities of fabric processing.

[0075] The model presented is based on the geometry of a random fiberweb made of rigid, straight, continuous fibers, of which a glass fibermat is a good example. This is perhaps the simplest, most open and idealweb geometry. Certainly many deviations from this ideal exist inpractice. A common deviation is due to non-random fiber depositionassociated with fiber bunching or clumping. As discussed by Davies(cited above), the resulting structure acts as if it is made of fibersof an effective fiber size somewhat larger than the actual fiber size.

[0076] Changes in fiber properties which affect how fibers pack in threedimensions, such as fiber shape, stiffness, crimp, etc, will result instructural deviations from the ideal. Also, the fluid-fiber wettingcharacteristics reflected in surface tension, σ, and the wetting angle,θ, may vary. In most cases this would reduce the maximum achievablehydrohead as per Equations 4 and 5. Hence, there can be other specificproperty balances and these are implicit in the scatter of the data inFIGS. 1 and 2 and the data set forth in Tables 4 and 5 below. Inprinciple the model can be refined for specific cases. Nevertheless, theanalysis of the ideal structure serves well as a benchmark and a guide.

[0077] The present invention is a nonwoven fabric comprising a supportweb and a barrier web, having a hydrohead of at least about 145 cm and aFrazier permeability of at least about 0.3 m³/m²-min. The nonwovenbarrier sheet can be hydrophobic, said hydrophobicity being derived fromeither coating a hydrophilic sheet with a hydrophobic coating material,such as a fluorocarbon- or silicone-based coating material, or byforming the sheet from hydrophobic polymers or copolymers, such aspolyolefins, including but not limited to those having repeating unitsderived from ethylene, propylene, butenes, hexenes, octenes, styrenes,4-methylpentene-1 and combinations thereof, and partially fluorinated orperfluorinated polymers or copolymers, including but not limited toethylene/tetrafluoroethylene (E/TFE), ethylene/chlorotrifluoroethylene,polyvinylidene fluoride (PVDF), fluorinated ethylene/propylene (FEP), acopolymer of tetrafluoroethylene and a perfluoro(alkyl vinyl ether)(PFA), and the like.

[0078] The diameter of the barrier web fibers is usually less than about2 micrometers, more usually less than about 1 micrometer and can even bein the “nanofiber” range, having diameters of less than about 0.5micrometer, wherein the diameter is the number average fiber size.

[0079] The fibers in the support webs for the barrier webs are usuallyless than 20 times, more usually less than 15 times and most usuallyless than 10 times the diameter of the corresponding barrier web fibers.For example, the support web fibers can have diameters greater thanabout 13 micrometers, which roughly corresponds to the diameter ofconventional spunbond fibers, about 12 micrometers or less, whichroughly corresponds to the diameter of micro-denier spunbond fibers, orabout 5 micrometers or less, which roughly corresponds to the diameterof melt blown fibers.

[0080] The support web can be any fabric which is configured to providesuitable support to the very fine fiber web. Among suitable support websare conventional spunbond and melt-blown webs, micro-denier spunbondwebs such as disclosed in U.S. Pat. No. 5,885,909, and variouscombinations of such different conventional nonwoven webs with one ormore of the very fine fiber webs.

[0081] It is also possible to provide a hydrophobic nonwoven sheetcontaining nanofibers according to the present invention by depositing anonwoven web of conventional hydrophilic polymer nanofibers onto acollecting/supporting web and coating the web's nanofibers with ahydrophobic coating material, such as a fluorocarbon coating material.When the coating material is applied in an extremely thin layer, littleif any change in the air permeability properties of the underlying webis caused, for example as described in co-pending U.S. provisionalapplication No. 60/391,864, filed 26 Jun. 2002.

[0082] In order to minimize air flow resistance and maximize flexibilityof the nonwoven fabrics of the invention, the support layer basis weightcan be less than 17 g/m², or less than 14 g/m², or less than 11 g/m², orless than 7 g/m², or less than 3 g/m², or even less than 1 g/m².

[0083] The nonwoven fabrics of the present invention have hydroheads ofat least about 145 cmwc and Fraziers of at least about 0.3 m³/m²-min, orat least about 1 m³/m²-min, or at least about 3 m³/m²-min, or at leastabout 5 m³/m²-min, or even at least about 10 m³/m²-min. The hydroheadsof the inventive fabrics can be greater than or equal to 150 cmwc,greater than or equal to 200 cmwc, greater than or equal to 300 cmwc, oreven greater than or equal to 400 cmwc.

[0084] The nonwoven fabrics of the present invention have maximum poresize between fibers, as measured by bubble point (ASTM E128), of lessthan about 23 micrometers, or less than about 20 micrometers, or lessthan about 15 micrometers, or even less than about 12 micrometers.

EXAMPLES

[0085] Sample fabrics were made by dissolving various polymers insuitable solvents which were then fed into an electrospinning apparatus,such as that described in U.S. Pat. No. 4,127,706, incorporated hereinby reference. The fine fibers formed were deposited onto a melt blownfabric support layer to form a barrier layer of the fine fibers, andmechanical strength was imparted to the samples by sandwiching the finefiber/melt blown layers between layers of spunbond polyester fibers, toform a four-layer laminate of spunbond/melt blown/fine fiber/spunbondconfiguration.

[0086] Fine fibers were spun from two different hydrophobic polymers:Kraton™ D1134x, a styrene-butadiene copolymer (specific gravity=0.94),available from Kraton™ Polymers of Houston, Tex.; and Kynar™ 761, apolyvinylidene fluoride polymer (specific gravity=1.76), available fromAtofina Chemicals, Inc. of Philadelphia, Pa. Kraton™ fine fibers werespun from solutions of 9wt. % polymer in a mixed solvent of 88/12 wt %tetrahydrofuran/dimethyl acetamide (THF/DMAC) and Kynar™ fine fiberswere spun from solutions of 14-15 wt. % polymer in acetone.

[0087] A Sage™ Model 362 syringe pump by Orion was used to pump solutionthrough a standard syringe with a blunt 27 gauge needle. High voltagewas supplied to the needle by inserting the needle through an insulatedaluminum foil strip connected to a Spellman SL300 negative high voltagepower supply. To assure reliable syringe pump operation in a highvoltage environment it was necessary to isolate the pump electricallyand then to ground the power supply reference lead, the metal case, andthe support jack of the syringe pump.

[0088] The deposition target was a brass disk 89 mm in diameter by 64 mmthick with a fully radiused edge. The disk was mounted on anelectrically insulating stand, for example, make of Lexan®, such that itwas suspended about 4 mm in front of the stand and connected via a screwthrough the stand to ground. A spunbond shroud (18 g/m² Remay polyester)covered the face of the disk and stand to keep fibers from accumulatingon the back side of the disk. A 76 mm diameter circle was cut out of theshroud over the face of the disk to expose the target area. A circularportion of melt blown substrate was mounted in the target area. ForKraton™ spins, uniform deposition of fibers was aided by insulating thetarget area with a polymeric film.

[0089] In one Kynar™ case (Example 12), a 15×15 cm fabric was made bydepositing fibers directly onto stainless steel cylinder 48 mm indiameter by 148 mm long. The melt blown layer was wrapped around thecylinder and the two layers were cut and peeled away to form the corelaminate.

[0090] Fabric properties were measured on 25 mm diameter circular areasof each fabric.

[0091] Air permeability and Bubble Point were measured on a PorousMedia, Inc. Capillary Flow Porometer, according to the principles ofASTM F778 and ASTM F316-03, respectively, and are reported as FrazierPermeability in units of m³/m²-min and pore size in micrometers,respectively.

[0092] Hydrohead measurements were run on an Aspull Mk III HydrostaticHead tester per method AATC TM 127, modified by using aluminum platesand an O-ring seal to hold the small fabric samples. Hydrohead wasrecorded at the first water penetration and is reported in centimetersof water column (cmwc).

[0093] Fine fiber loading was measured gravimetrically by the massdifference of the sample before and after fine fiber deposition, and isreported as an average over the surface area of the sample (total gramsfine fibers deposited/sample area).

Control Examples

[0094] Three control examples were made of spunbond/melt blown/spunbondconstruction, wherein the spunbond layers were 18 g/m² polyester(polyethylene terephthalate) and the melt blown layers were 18 g/m²bicomponent 65 wt. % polyester/35 wt. % polyethylene fibers madeaccording to the description of WO 01/09425 A1, incorporated herein byreference. The control fabrics were prepared in the same way as theexemplary fabrics, except that no fine fiber layer was deposited. TABLE3 Frazier Hydrohead Control Example # (m³/m²-min) (cmwc) 1 31.4 29 2 6.757 3 7.7 56

[0095] A number of electrospinning runs were conducted in order todetermine the most effective combination of polymer, solvents, andconcentrations, as well as uniform deposition and handling techniques,to make the fine fiber barrier layers of the present invention. Datafrom the best combination of electrospinning parameters and collectiontechniques determined is set forth below.

EXAMPLES 1-9

[0096] Kraton™ D1133×copolymer was dissolved in a mixed solvent of 88wt. % tetrahydrofuran/12 wt. % dimethyl acetamide at a polymerconcentration of 9 wt. %, and electrospun at −18 KV at a rate of 0.5ml/hr. Fine fibers were deposited onto samples of 18 g/m² bicomponentmelt blown fabric described in the Control Examples at a collectiondistance of approximately 22 cm. The fine fiber layer was then coveredwith a layer of spunbonded polyester, removed from the sample target.The layer of melt blown collection fabric was also covered with a layerof spunbonded polyester and all four layers were consolidated into alaminate. The barrier properties of the Examples were measured and arereported below in Table 4.

[0097] The fine fibers collected were measured by scanning electronmicroscopy and found to have diameters in a general range of betweenabout 0.1 to 1.8 micrometers, with the average fiber diameters believedto be less than about 1 micrometer. TABLE 4 Fiber load Frazier HydroheadBubblepoint Example # (g/m²) (m³/m²-min) (cmwc) (micrometers) 1 5* 0.4222 — 2 2.5 1.6 79 — 3 11 .24 37 — 4 4.8 0.6 86 — 5 1.5 3.2 26 — 6 2.61.0 92 — 7 6.0 0.7 118 — 8 1.5 3.8 105 — 9 12.8 0.3 128 21.6

[0098] In some cases the fine fibers were observed to shrink and crackupon drying. While the reasons for the data inconsistencies are notfully understood, it is believed that the relative humidity of the airduring sample formation and collection drastically affects the spinningprocess and ultimately the barrier properties of thestyrene-butadiene/THF/DMAC system, which makes it difficult to obtainuniform fiber deposition across the surface of the collection webs andbarrier properties.

EXAMPLES 10-19

[0099] Kynar™ polymer was dissolved in acetone solvent at a polymerconcentration of 15 wt. %, and electrospun at −20 KV at a rate of 5ml/hr. Fine fibers were deposited onto samples of 18 g/m² bicomponentmelt blown fabric described in the Control Examples at a collectiondistances of approximately 22-30 cm. The fine fiber layer was thencovered with a layer of spunbonded polyester, removed from the sampletarget. The layer of melt blown collection fabric was also covered witha layer of spunbonded polyester and all four layers were consolidatedinto a laminate. The barrier properties of the Examples were measuredand are reported below in Table 5.

[0100] The fine fibers collected were measured by scanning electronmicroscopy and found to have diameters in a general range of betweenabout 0.14 to 2.8 micrometers, with the average fiber diameters believedto be less than about 1 micrometer. TABLE 5 Fiber load Frazier HydroheadBubblepoint Example # (g/m²) (m³/m²-min) (cmwc) (micrometers) 10 13.611.2 131 11.3 11 22 3.1 115 10.9 12 16.5 1.0 278 10.7 13 ″ 2.1 347 — 1451.1 0.7 399 9.4 15 22.7 1.6 345 11.8 16 26 1.0 368 8.0 17 15.4 1.5 3228.7 18 23.9 0.7 332 4.3 19 20.0 0.8 279 5.8

[0101] Example 13 was a portion of the fabric sample of Example 12 whichwas calendered using a metal roll on a metal plate with a linearpressure estimated to be about 2-4 kg/cm.

[0102] Overall, the barrier fabrics containing the Kynar™ fine fibersexhibited much greater hydrohead values than those of either the ControlSamples or Examples 1-9. It is believed that the more hydrophobic natureof the polyvinylidene fluoride polymer in Examples 10-19, relative tothe styrene-butadiene polymer of Examples 1-9, is a major reason for theimproved hydrohead values. However, those of skill in the art willrecognize that the hydrohead of the styrene-butadiene polymer fabrics ofExamples 1-9 could be enhanced by treatment with a water-repellantchemical finish, such as a fluorochemical finish, without appreciabledetriment to the Frazier permeability of the fabrics.

[0103] Further, it is important to note that in almost all cases, thehydrohead measurements of the fine fiber-containing exemplary fabrics ofthe present invention exceed those of the Control Examples, which areessentially spunbond/melt blown/spunbond fabric construction. Thisdemonstrates that the presence of a fine fiber layer, especially whereinthe fine fibers comprise fibers of less than about 2 micrometers indiameter, or even less than about 1 micron in diameter, can greatlyenhance liquid barrier properties of a fabric.

[0104] The laminate fabric configurations, Fine Fiber barrierlayer/spunbond support layer (FF/SB) and spunbond support layer/FineFiber barrier layer/spunbond support layer (SB/FF/SB) are viableconfigurations for achieving higher barrier with air permeability belowabout Frazier=m³/m²-min. Typical spunbond fiber diameter size is 10micrometers and above.

[0105] Suitable support layers must have pore sizes scaled to themechanical strength of the barrier layer. The weaker the barrier layer,the smaller the support layer pore size must be for adequate support.Smaller pores sizes, in turn, require smaller fiber diameter sizes.Hence, as barrier layer basis weight is reduced to facilitate high airpermeability, suitable support layers must have fiber diameter sizessmaller than typical spunbond fiber sizes.

[0106] Such smaller fibers could be micro-denier spunbond (mSB), asdiscussed in U.S. Pat. No. 5,885,909, e.g., 6<D_(f)<10 micrometers whichare strong enough to meet the mechanical strength requirements for thefabric as a whole. Micro-denier spunbond support would give rise to twofabric configurations: FF/mSB and mSB/FF/mSB.

[0107] Non-self-supporting support layers with fiber diameters in therange of 1<D_(f)<10 micrometers can be made by the melt blowing process.Typically these fibers are not strong (0.3<GPD<0.6). They are used toprovide barrier properties with a support layer of spunbond fibers toprovide strength. If melt blown fibers are used to support the FineFiber barrier layer, the melt blown fiber layer still requires a supportlayer to maintain over all fabric mechanical strength. A spunbond fiberlayer is well suited to be the support layer.

[0108] This gives rise to the laminate fabric configurations: FF/MB/SB,SB/MB/FF/MB/SB, FF/MB/mSB and mSB/MB/FF/MB/mSB.

[0109] There can be asymmetrical combinations of these layer types,e.g., SB/FF/MB/SB, which could have asymmetrical barrier performance,which might provide unusual but useful function to fabrics of suchconstructions. For example, if the liquid challenge is from the SB/FFside, the barrier will be high and equal to the maximum barriercapability of the FF layer. If the liquid challenge is from the SB/MBside, the spunbond layer will not provide adequate support for the FFlayer which will break at some hydrohead lower than FF layer capability.

We claim:
 1. A nonwoven fabric comprising a support web and a fibrousbarrier web, having a hydrohead of at least about 145 cm and a Frazierpermeability of at least about 0.3 m³/m²-min.
 2. A nonwoven fabriccomprising at least one support web and a hydrophobic barrier web withfibers having diameters of less than 2.0 micrometers, a hydrohead of atleast about 145 cm and a Frazier permeability of at least about 0.3m³/m²-min.
 3. The nonwoven fabric of claims 1 or 2, wherein said barrierweb fibers have diameters of less than 1.0 micrometer.
 4. The nonwovenfabric of claims 1 or 2, wherein said barrier web fibers have diametersof less than 0.5 micrometer.
 5. The nonwoven fabric of claim 3, whereinthe barrier layer basis weight is no more than 15 g/m².
 6. The nonwovenfabric of claim 4, wherein the barrier layer has a basis weight of nomore than 3 g/m².
 7. The nonwoven fabric of claims 1 or 2, wherein saidbarrier web comprises nanofibers of hydrophobic polymer or copolymer. 8.The nonwoven fabric of claim 7, wherein said hydrophobic polymer orcopolymer is a polyolefin, a partially fluorinated polymer or aperfluorinated polymer.
 9. The nonwoven fabric of claim 8, wherein saidhydrophobic polymer or copolymer has repeating units derived fromethylene, propylene, butenes, hexenes, octenes, styrenes,4-methylpentene-1 and combinations thereof.
 10. The nonwoven fabric ofclaims 1 or 2, wherein said barrier web is rendered hydrophobic bycoating with a hydrophobic coating.
 11. The nonwoven fabric of claim 10,wherein said hydrophobic coating is a fluorocarbon coating material. 12.The nonwoven fabric of claims 1 or 2, wherein the barrier web has amaximum pore size between fibers of no more than about 23 micrometers.13. The nonwoven fabric of claims 1 or 2, wherein the barrier web has asolids fraction of no less than about 0.03.
 14. A nonwoven barrierfabric comprising a fibrous barrier web, said fabric having a hydroheadof at least about 145 cm and a Frazier permeability of at least about0.3 m³/m²-min and having a relationship between barrier web basisweight, and fabric hydrohead and Frazier permeability described by theformula:${{{Bwt}\quad \left( {g\text{/}m^{2}} \right)} \leq \frac{4000 \cdot c \cdot \left( {1 - {2.3 \cdot c}} \right) \cdot \rho_{f}}{{Frazier} \cdot {Hydrohead}^{\quad {k{(c)}}}}},$

wherein ρ_(f), is the density of the barrier fibers, kg/m³c is thesolids volume fraction of the barrier web, k(c)=3.58·c ²−1.32·c+1.77,Frazier is in units of m³/m²-min, and Hydrohead is in units ofcentimeters of water column.
 15. A nonwoven fabric according to one ofclaims 1, 2 or 14, comprising a structure of FF/mSB, wherein FF is abarrier web.
 16. A nonwoven fabric according to one of claims 1, 2 or14, comprising a structure of FF/SB, wherein FF is a barrier web.
 17. Anonwoven fabric according to one of claims 1, 2 or 14, comprising astructure of mSB/FF/mSB, wherein FF is a barrier web.
 18. A nonwovenfabric according to one of claims 1, 2 or 14, comprising a structure ofFF/MB/SB, wherein FF is a barrier web.
 19. A nonwoven fabric accordingto one of claims 1, 2 or 14, comprising a structure of SB/MB/FF/MB/SB,wherein FF is a barrier web.
 20. A nonwoven fabric according to one ofclaims 1, 2 or 14, comprising a structure of FF/MB/mSB, wherein FF is abarrier web.
 21. A nonwoven fabric according to one of claims 1, 2 or14, comprising a structure of mSB/MB/FF/MB/mSB, wherein FF is a barrierweb.
 22. A nonwoven fabric according to one of claims 1, 2 or 14,comprising a structure of SB/MB/FF/SB, wherein FF is a barrier web. 23.A nonwoven fabric of claims 1 or 2, wherein said support web comprisesfibers having diameters less than about 20 times the barrier web fiberdiameters.
 24. The nonwoven fabric of claim 23, wherein said support webfibers have diameters less than about 13 micrometers.